Re: [PATCH] arm64: NUMA: Kconfig: Increase max number of nodes

From: Robin Murphy
Date: Thu Oct 22 2020 - 07:21:33 EST

On 2020-10-22 02:07, Vanshi Konda wrote:
On Thu, Oct 22, 2020 at 12:44:15AM +0100, Robin Murphy wrote:
On 2020-10-21 12:02, Jonathan Cameron wrote:
On Wed, 21 Oct 2020 09:43:21 +0530
Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:

On 10/20/2020 11:39 PM, Valentin Schneider wrote:

Hi,

Nit on the subject: this only increases the default, the max is still 2?????.

Agreed.


On 20/10/20 18:34, Vanshidhar Konda wrote:
The current arm64 max NUMA nodes default to 4. Today's arm64 systems can
reach or exceed 16. Increase the number to 64 (matching x86_64).

Signed-off-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
---
 arch/arm64/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 893130ce1626..3e69d3c981be 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -980,7 +980,7 @@ config NUMA
 config NODES_SHIFT
      int "Maximum NUMA Nodes (as a power of 2)"
      range 1 10
-    default "2"
+    default "6"

This leads to more statically allocated memory for things like node to CPU
maps (see uses of MAX_NUMNODES), but that shouldn't be too much of an
issue.

The smaller systems should not be required to waste those memory in
a default case, unless there is a real and available larger system
with those increased nodes.


AIUI this also directly correlates to how many more page->flags bits are
required: are we sure the max 10 works on any aarch64 platform? I'm

We will have to test that. Besides 256 (2 ^ 8) is the first threshold
to be crossed here.

genuinely asking here, given that I'm mostly a stranger to the mm
world. The default should be something we're somewhat confident works
everywhere.

Agreed. Do we really need to match X86 right now ? Do we really have
systems that has 64 nodes ? We should not increase the default node
value and then try to solve some new problems, when there might not
be any system which could even use that. I would suggest increase
NODES_SHIFT value upto as required by a real and available system.

I'm not going to give precise numbers on near future systems but it is public
that we ship 8 NUMA node ARM64 systems today.  Things will get more
interesting as CXL and CCIX enter the market on ARM systems,
given chances are every CXL device will look like another NUMA
node (CXL spec says they should be presented as such) and you
may be able to rack up lots of them.

So I'd argue minimum that makes sense today is 16 nodes, but looking forward
even a little and 64 is not a great stretch.
I'd make the jump to 64 so we can forget about this again for a year or two.
People will want to run today's distros on these new machines and we'd
rather not have to go around all the distros asking them to carry a patch
increasing this count (I assume they are already carrying such a patch
due to those 8 node systems)

To echo Jonathan's statement above we are looking at systems that will
need approximately 64 NUMA nodes over the next 5-6 years - the time for
which an LTS kernel would be maintained. Some of the reason's for
increasing NUMA nodes during this time period include CXL, CCIX and
NVDIMM (like Jonathan pointed out).

The main argument against increasing the NODES_SHIFT seems to be a
concern that it negatively impacts other ARM64 systems. Could anyone
share what kind of systems we are talking about? For a system that has
NEED_MULTIPLE_NODES set, would the impact be noticeable?

Systems like the ESPRESSObin - sure, sane people aren't trying to run desktops or development environments in 1GB of RAM, but it's not uncommon for them to use a minimal headless install of their favourite generic arm64 distro rather than something more "embedded" like OpenWrt or Armbian. Increasing a generic kernel's memory footprint (and perhaps more importantly, cache footprint) more than necessary is going to have *some* impact.

Robin.


Vanshi


Nit: I doubt any sane distro is going to carry a patch to adjust the *default* value of a Kconfig option. They might tune the actual value in their config, but, well, isn't that the whole point of configs? ;)

Robin.


Jonathan


      depends on NEED_MULTIPLE_NODES
      help
        Specify the maximum number of NUMA Nodes available on the target

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